Cobaltocene/cobaltocenium

Rogers El, Silvester DS, Poole DL, Aldous L, Hardacre C, Compton RG (2008) Voltammetric characterization of the ferrocene ferrocenium and cobaltocenium cobaltocene redox couples in RTlLs. J Phys Chem C 112 2729-2735 Scott MP, Brazel CS, Benton MG, Mays JW, Holbrey JD, Rogers RD (2002) Application of ionic liquids as plasticizers for poly(methyl methacrylate). Chem Commun 2002 1370-1371 Seddon KR (1997) Ionic liquids for clean technology. J Chem Tech Biotechnol 68 315-316... 

Rogers El, Silvester DS, Poole DL, Aldous L, Hardacre C, Compton RG (2008) Voltammetric characterization of the ferrocene/ferrocenium and cobaltocenium/cobaltocene redox couples in RTILs. J Phys Chem C 112(7) 2729-2735... 

The observation of radical anions has been confirmed by ESR measurements as illustrated by [Ir4(CO)12], (g = 2.002) 208). Similarly, a toluene solution of Co4(CO)i2 reacts with cobaltocene precipitating a brown compound which is extremely reactive and contains a cobaltocenium cation for each four cobalt atoms of the anion55. With excess cobaltocene (or alkali metals) in THF the reaction proceeds further as shown in Eq. (20),... 

In a similar manner to manganocene, cobaltocene is easily oxidized to the corresponding cobaltocenium ion [Co(f/5-C5H5)2]+, which, as illustrated in Figure 48, exhibits two distinct chemically reversible reductions corresponding to Coni/Con and Con/CoI.88... 

It is useful to note that in contrast to the elongation of the M-C bonds observed on passing from ferrocene to ferrocenium ion, in the case of the cobaltocene/cobaltocenium transition, because of the removal of... 

The ferrocene/ferrocenium reference redox system at platinum fulfills these requirements fairly well [4-6]. Another system which has been recommended is bis(biphenyl)chromium (0)/bis(biphenyl)chromium (+1) (BCr+ /BCr) [5, 7]. Several other systems have been suggested, and used sporadically, such as cobaltocene/cobaltocenium, tris(2,2 -bipyridine) iron (I)/tris(2,2 -bipyridine) iron (0), Rb+/Rb(Hg), and so on. 

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

Dendrimers Terminated with Cobaltocenium and Ferrocene-Cobaltocenium Units Like ferrocene, cobaltocenium is an excellent organometallic moiety to incorporate in or functionalize dendritic systems. As already discussed, it is indeed isoelectronic with ferrocene, highly stable, positively charged complex, which undergoes a reversible monoelectronic reduction to yield the neutral cobaltocene. 

The 18-electron rale is not obeyed as consistently by these types of oiganome-tank compounds a by the carbonyl and nitrosyl complexes and their derivatives. For example, in addition to ferrocene. M(i 5-CsHs)2 compounds are known for most of the other elements of the first transition series (M — V, Cr, Mn.Co, Ni) and these cannot obey ihe 18-electron rule. However, only ferrocene shows exceptional thermal stability (stable to 500 C) and is not oxidized by air. Furthermore, cobaltocene, a 19-electron species, is readily oxidized to the 18-electron cobaltocenium ion. (Co(ip-CsH )3)4 , which reflects much of the thermal stability of ferrocene. Mixed cyclopentadienyl carbonyl complexes are common K -CjHjMCO) ]. [(if-CjH )-Cr(CO), . [( -CjHOMnCCOjJ, [(>r-C,H,>Fe(CO ,, . [fo -CjiyCoCoy. and (ip-CsH,)Ni(CO) 2. Of interest is the fact that among these compounds, the odd-atomic-number elements (V. Mn, and Co) form monomers and the even-atomic-number elements (Cr. Fe. and Ni) Ibrm dimers, which is in direct contrast to the behavior shown by the simple carbonyl complexes. Cyclopentadienyl derivatives are now known for every main group and transition metal of the periodic table and for most of the -block metals.89... 

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